When a user left his residence with a door or a window opened, an unauthorized party might break into his residence through an opened door or window, causing serious property damages. Accordingly, it is desirable if the state of the door and window could be acquired after the user left his residence and the user could be prompted when the door or window is opened.
A method for sending a message in the related art includes: installing a sensor in advance on the door and window; and setting an absent time period when the user will not be in the residence. A server detects whether the door or window is opened during the absent time period. If the door or the window is opened during the absent time period, the server sends a prompt message to a terminal device of the user.